Cyanobacteria and purple sulfur bacteria. The major photosynthetic groups of bacteria have formed stable ecological associations for billions of years. The discovery that anoxygenic photosynthetic bacteria existed before cyanobacteria awakens memories of an ancient world, perhaps earlier than 3500 Ma, where all photosynthetic ecosystems required reduced gases and solutes from geothermal activity for their sustenance.

We know little about the origins of one of life's most fundamental mechanisms--the ability to harness the sun's energy through photosynthesis. But a paper[2] in this week's issue of Science provides a new jumping-off point by identifying what appears to be the earliest organism to use the mechanism.

The surprising winner in the photosynthetic sweepstakes is purple bacteria. They belong to one of five groups whose genes were examined by Carl Bauer, a biologist at Indiana University in Bloomington, and his colleagues. Bauer wanted to know which genes most closely resembled each other and which had more mutations. The more disparate the genes, he reasoned, the farther apart in age the groups would be. Looking at more than 100 genes, the researchers found that the purple bacteria were oldest, followed by sulfurous and nonsulfurous green bacteria, heliobacteria, and oxygen-producing cyanobacteria. "Now we can key in on the purple bacteria to see how photosynthesis works on its oldest and most fundamental level and, maybe, how it began," Bauer says.

The finding overturned the accepted wisdom that the title of oldest light-absorbing organism belonged to the green bacteria, believed to be about 3.5 billion years old. By comparison, fossils of cyanobacteria, formerly known as blue-green algae, have been dated at 2.5 billion years.

"This is the first time anyone has put together such a complete profile of genes from all the photosynthetic groups of bacteria," says Robert Blankenship, a biochemist at Arizona State University in Mesa. "Bauer and colleagues have advanced the ball considerably."